<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <style>
        div{
            width: 100px;
            height: 100px;
            border: 2px solid red;
            /*border-radius: 20px;  !*上下左右都是20px*!*/
            /*border-radius: 20px 50px;  !*左上和右下20px，左下和右上50px*!*/
        }
    </style>

</head>
<body>

<div></div>

</body>
</html>